SLANT 6 a GAS GUZZLER?

I think a more typical mileage figure (on a well-worn, non-rebuilt, at best averagely maintained slant6- in other words, almost all of them) would be in the high teens.. say 17 mpg.
my 68 /6 has 103,000 actual miles and gets 20.7mpg on short trips/intown driving..dont never go to far,so not sure what the hiway mileage would be..